1. Field of the Invention
The present invention relates to a flow dividing pump which divides and supplies oil to two separate units.
2. Description of the Related Art
A conventional variable capacity pump is disclosed in Japanese Unexamined Patent Publication No. 61-155677. In order to divide the flow from this type of pump a flow divider unit is attached to the pump. Such flow dividers are conventionally used in industrial vehicles, as for example to divide the hydraulic fluid flow between a power steering device and a loading hydraulic cylinder.
The pump with such a unit has an end plate securely attached to one end of the pump housing. The end plate has a common discharge passage formed therein and is attached with the flow divider unit. The flow divider unit includes an orifice which communicates with the discharge passage and a spool valve (flow divider) which controls the flow rate of oil in cooperation with the orifice. An oil discharge passage and an excess oil discharge passage which communicate with the orifice are provided in the flow divider unit.
The oil discharge passage (hereafter referred to as "control passage") is connected via, for example, a check valve to a power steering device in a forklift truck. The excess oil discharge passage (hereafter referred to as "excess passage") is connected via, for example, a control valve to a hydraulic cylinder in a loading device.
If the flow rate of oil discharged from the pump exceeds a predetermined control flow rate, the difference between the pressures upstream and downstream of the orifice in the flow divider unit becomes large, shifting the spool valve. This shift of the spool valve narrows the opening of the control passage, and forces excess oil to flow into the excess passage so as to maintain the constant flow rate of oil to the power steering device.
When the power steering device is actuated, the spool valve is controlled to open the control passage wider and narrow the opening of the excess passage. On the other hand, when the loading hydraulic cylinder is actuated, the spool valve is adjusted to open the excess passage wider and narrow the opening of the control passage.
Since the flow divider unit is attached to the end plate of the housing of the flow dividing pump described above, the number of necessary components increases, making the whole pump larger and heavier.
Further, the oil passage in this pump must be relatively long in order to feed oil through the end plate to the flow divider unit. This results in an oil pressure loss, which decreases the pumping efficiency. In addition, oil will be heated while passing through the oil passage and become less viscous, which may result in excessive supply of the oil to the downstream side.
For an average forklift truck, the maximum pressure of oil in the power steering device is 85 kgf/cm.sup.2, whereas the maximum oil pressure in the loading hydraulic cylinder is 185 kgf/cm.sup.2, which is very high. The temperature of oil particularly in the hydraulic cylinder side is likely to rise, and excess oil tends to be supplied to the hydraulic cylinder rather than to the power steering device.